Methods for the quantification of nucleic acids are important in many areas of molecular biology and in particular for molecular diagnostics. At the DNA level such methods are used for example to determine the copy numbers of gene sequences amplified in the genome. However, methods for the quantification of nucleic acids are used especially in connection with the determination of mRNA quantities since this is usually a measure for the expression of the respective coding gene.
If a sufficient amount of sample material is available, special mRNAs can be quantified by conventional methods such as Northern Blot analysis or RNAse protection assay methods. However, these methods are not sensitive enough for sample material that is only available in small amounts or for genes that are expressed very weakly.
The so-called RT-PCR is a much more sensitive method. In this method a single-stranded cDNA is firstly produced from the mRNA to be analysed using a reverse transcriptase. Subsequently a double-stranded DNA amplification product is generated with the aid of PCR.
A distinction is made between two different variants of this method:                In the so-called relative quantification the ratio of the expression of a certain target RNA is determined relative to the amount of RNA of a so-called housekeeping gene which is assumed to be constitutively expressed in all cells independent of the respective physiological status. Hence the mRNA is present in approximately the same amount in all cells.        
The advantage of this is that different initial qualities of the various sample materials and the process of RNA preparation has no influence on the particular result. However, an absolute quantification is not possible with this method.                Alternatively the absolute amount of RNA used can be determined with the aid of standard nucleic acids of a known copy number and amplification of a corresponding dilution series of this standard nucleic acid. There are two alternatives:        
When using external standards the standard and target nucleic acid are amplified in separate reaction vessels. In this case a standard can be used with an identical sequence to the target nucleic acid. However, systematic errors can occur in this type of quantification if the RNA preparation to be analysed contains inhibitory components which impair the efficiency of the subsequent PCR reaction. Such errors can be excluded by using internal standards i.e. by amplifying the standard and target nucleic acid in one reaction vessel. However, a disadvantage of this method is that standards have to be used that have different sequences compared to the target nucleic acid to be analysed in order to be able to distinguish between the amplification of the standard and target nucleic acid. This can in turn lead to a systematic error in the quantification since different efficiencies of the PCR amplification cannot be excluded when the sequences are different.
PCR products can be quantified in two fundamentally different ways:
a) End Point Determination of the Amount of PCR Product Formed in the Plateau Phase of the Amplification Reaction
In this case the amount of PCR product formed does not correlate with the amount of the initial copy number since the amplification of nucleic acids at the end of the reaction is no longer exponential and instead a saturation is reached. Consequently different initial copy numbers exhibit identical amounts of PCR product formed. Therefore the competitive PCR or competitive RT-PCR method is usually used in this procedure. In these methods the specific target sequence is coamplified together with a dilution series of an internal standard of a known copy number. The initial copy number of the target sequence is extrapolated from the mixture containing an identical PCR product quantity of standard and target sequence (Zimmermann and Mannhalter, Bio-Techniques 21:280-279, 1996). A disadvantage of this method is also that measurement occurs in the saturation region of the amplification reaction.
b) Kinetic Real-Time Quantification in the Exponential Phase of PCR.
In this case the formation of PCR products is monitored in each cycle of the PCR. The amplification is usually measured in thermocyclers which have additional devices for measuring fluorescence signals during the amplification reaction. A typical example of this is the Roche Diagnostics LightCycler (Cat. No. 2 0110468). The amplification products are for example detected by means of fluorescent labelled hybridization probes which only emit fluorescence signals when they are bound to the target nucleic acid or in certain cases also by means of fluorescent dyes that bind to double-stranded DNA. A defined signal threshold is determined for all reactions to be analysed and the number of cycles Cp required to reach this threshold value is determined for the target nucleic acid as well as for the reference nucleic acids such as the standard or housekeeping gene. The absolute or relative copy numbers of the target molecule can be determined on the basis of the Cp values obtained for the target nucleic acid and the reference nucleic acid (Gibson et al., Genome Research 6:995-1001; Bieche et al., Cancer Research 59:2759-2765, 1999; WO 97/46707; WO 97/46712; WO 97/46714).
In summary in all the described methods for the quantification of a nucleic acid by PCR the copy number formed during the amplification reaction is always related to the copy number formed of a reference nucleic acid which is either a standard or an RNA of a housekeeping gene. In this connection it is assumed that the PCR efficiency of the target and reference nucleic acid are not different.
Usually a PCR efficiency of 2.00 is assumed which corresponds to a doubling of the copy number per PCR cycle (User Bulletin No. 2 ABI Prism 7700, PE Applied Biosystems, 1997).
However, it has turned out that the real PCR efficiency can be different from 2.00 since it is influenced by various factors such as the binding of primers, length of the PCR product, G/C content and secondary structures of the nucleic acid to be amplified and inhibitors that may be present in the reaction mixture as a result of the sample preparation. This is particularly relevant when using heterologous reference nucleic acids e.g. in the relative quantification compared to the expression of housekeeping genes.